Lens Formula & Power of the Lens

The relationship between the object distance (u), the image distance (v) and the focal length (f) of the lens is called lens formula.

From the above formula, Focal length of a lens is obtained as follows:

The power of a lens is the measure of its ability to produce convergence or divergence of a parallel beam of light. The power of a lens depends on its focal length. The power of a lens is defined as the reciprocal of its focal length in metres. The unit of power of a lens is dioptre (D).

If two lenses of focal length f₁ and f₂ are in contact, then:

And

P = P₁ +P₂

For example, if a lens has a focal length of 40 cm, its power would be: 1/40 x10^-2= 100/4 = 2.5D